A suction of backfill sand behind a seawall causes damages of seawall, such as local subsidence of seawall or cave-in of backfill sand. Some field observations show that caves were formed due to suction through a corroded hole found in a sheet pile under the seawall. Therefore, in this study, a numerical simulation on such a cave-formation process is carried out by a particle method. The simulation consists of two stages: The one is a simulation of a suction process of sand under the seawall, and the other is an erosion process of backfill sand. The cave reproduced by the simulation showed good agreement with the observed one.